1. Field of the Invention
The present invention relates to an ink-jet recording apparatus and an ink-jet recording system configured to record an image on a recording medium by discharging ink, measure colors of the recorded image, and generate color correction data based on a result of the measurement. More particularly, the present invention relates to an ink-jet recording apparatus and an ink-jet recording system having an overlapping head including a plurality of recording head chips overlapped to each other.
2. Description of the Related Art
There are many known recording methods for image recording apparatuses such as printers. In particular, printers using an ink-jet recording method are widely used in many applications varying from those for consumer use to those for industrial use such as a large-format printer. In a known type of ink-jet recording apparatus, ink is discharged from a recording head having a discharge port array including a plurality of discharge ports so as to record an image on a recording medium.
This type of ink-jet recording apparatus uses a method such as a serial recording method in which recording is performed while scanning a recording head over a recording medium, a full-line recording method in which a recording medium is conveyed with respect to a full-line head including a plurality of recording head chips arranged in an array, etc. Among those methods, the full-line recording method has a feature that the full-line recording head is capable of recording an image over the full width of a recording medium and thus recording is possible simply by conveying the recording medium in a direction crossing a direction in which a discharge port array is arranged, which makes it possible to perform the recording at a high speed.
In such a full-line recording head, it is possible to achieve a great recordable width by overlapping recording head chips for the same color such that an end region of each recording head chip is overlapped to an end region of an adjacent recording head chip. In the present description, a full-line head including a plurality of recording head chips whose end portions are overlapped to each other is referred to as an “overlapping head”. In general, the overlapping head is constructed such that adjacent recording head chips partially overlap each other, and image data to be recorded is distributed in each overlapping region of recording head chips.
A positioning error occurs when recording head chips are assembled together in production of a head, and the positioning error can produce color unevenness, streaks, etc., which can cause degradation in image quality. Several techniques have been proposed to deal with the image quality degradation. For example, Japanese Patent Laid-Open No. 2007-152582 discloses a technique in which a gradation mask is used to gradually distribute recording data over an overlapping region of each recording head chip of an overlapping head so that white or black streaks occurring in overlapping regions are shaded.
It is also known that a difference in size of ink dots discharged from discharge ports can produce streaks or color unevenness, which can cause degradation in image quality. The variation in ink dot size can occur due to several factors. For example, one factor is a variation in discharge port diameter that can occur when recording head chips are produced. Another factor is a variation in discharge energy, which can cause an error in dot size. To handle this situation, Japanese Patent No. 3040433 discloses a technique using head shading. More specifically, in this technique, a predetermined specific pattern is recorded on a recording medium and the recorded pattern is read to detect unevenness in recorded color density for each nozzle (discharge port). The unevenness in density is compensated for based on the detection result.
The present applicant has found that positioning errors in assembling of recording head chips can not only produce white/black streaks but also can cause other degradation in image quality. More specifically, positioning errors of recording head chips can cause ink droplets to hit wrong positions on a recording medium, which can cause a recorded image to have a difference in color (in hue, saturation, brightness, etc.) between an area recorded by a overlapping region and an area recorded by a non-overlapping region.
In this case, even when the recorded image has the same color for all areas recorded by non-overlapping regions of the recording head chips, the areas recorded by the overlapping regions have colors different from the color of the areas recorded by the non-overlapping regions, as described in further detail below with reference to FIGS. 2A to 2D.
FIG. 2A illustrates recording head chips A and B positioned with a slight positioning error. Note that these two recording head chips are assumed to discharge the same amount of ink with the same color, and the same color is recorded by the non-overlapping regions of the two recording head chips. FIG. 2B and FIG. 2C illustrate images recorded by the overlapping region. The image shown in each figure is of one pixel including nine dots. The gray level of the one pixel is represented by a combination of these nine dots. In the recording of these pixels shown in FIGS. 2A and 2B, image data is distributed to the recording head chip A and the recording head chip B, and ink is discharged at all nine dots.
FIG. 2B illustrates a comparative example in which the image is recorded by an overlapping region having no positioning error. FIG. 2C illustrates an example in which the image is recorded by an overlapping region having a positioning error such as that shown in FIG. 2A. In the example shown in FIG. 2B, ink dots in the pixel are formed at correct positions without overlapping each other. In contrast, in the example shown in FIG. 2C, ink dots in the pixel are formed at wrong positions due to the positioning error and thus overlapping occurs between dots formed on the recording medium by ink discharged by the recording head chip A and dots formed by ink discharged by the recording head chip B. The color of the image varies depending on a dot covering ratio on the recording medium and an overlapping ratio, and thus the color of the pixel shown in FIG. 2C is different from that shown in FIG. 2B although the pixel equally has nine dots.
In the case where there is no assembling error as in the example shown in FIG. 2B, the color is the same for the non-overlapping region and the overlapping region. However, if there is a positioning error as in the example shown in FIG. 2C, a difference in color occurs between the non-overlapping region and the overlapping region. If an image includes a plurality of such pixels located close to each other, the result is a great difference in color, which cannot be neglected.
The difference in color of the overlapping region from the non-overlapping region can occur in all overlapping regions. That is, in the case where recording is performed using the recording head having the structure realized by overlapping a plurality of recording head chips for the same color as with an example shown in FIG. 2D, a color shift due to an ink dot positioning error occurs at as many locations as there are overlapping regions (equal to the number of recording head chips minus one). The color shift can cause the recorded image to have a visually perceivable streak with a color different from the color of areas recorded by the non-overlapping regions, which results in degradation in image quality. To assemble the recording head chips such that no color shift occurs, the recording head chips may be assembled with high positioning accuracy, or high-precision recording head chips, which are expensive semiconductor devices, may be selected according to necessary characteristics, which can lead to a reduction in production yield. However, this leads to a great increase in cost.
The color shift due to overlapping of dots is difficult to avoid even when the gradation mask disclosed in Japanese Patent Laid-Open No. 2007-152582 is used. As described above, in the technique using the gradation mask, distribution ratios of discharge ports are gradually distributed in the overlapping region. That is, in a case where the overlapping region is not large enough to allow the process using the gradation mask to provide a sufficient effect as in a case where the overlapping region includes, for example, only one discharge port array, it is difficult to reduce the color shift due to overlapping of ink dots caused by ink dot positioning errors.
It is also difficult to avoid the color shift of this type by using the head shading technique disclosed in Japanese Patent No. 3040433, because head shading is a technique to correct an error of recording density due to an error of the amount of recording material discharged. That is, the color shift due to overlapping of dots caused by dot positioning errors is not a color shift caused by an error of recording density of a color and thus it is difficult to make a correction by the head shading technique disclosed in Japanese Patent No. 3040433.